The present invention relates to a method and apparatus for transmission and reception which simultaneously permit highly sophisticated channel and extremely suppressed peak power caused by multiplexing of plural modulated signals.
In future telecommunications, to provide advanced multimedia services of offering video, audio, or database contents is expected. In transmission techniques for telecommunications improvement of channel quality and improvement of transmission speed are indepensable to establish such services.
With a view to enhancing the channel quality, there have been proposed a method of increasing the signal energy on the channel, a method of utilizing channel coding, and a method of using diversity reception.
To improve of transmission speed, it is possible to use a method of speeding up the code transmission rate and a method of utilizing high efficiency modulation.
As transmission techniques that satisfies the both requirements of enhancement of the channel quality and the transmission speed, there are available several methods, and from the viewpoint of circuit technology, parallel transmission technique using an error correcting scheme is effective. The usefulness of the error correcting scheme decreases required Eb/NO (the energy per bit-to-noise spectral density ratio), making it possible to economize devices used and increase the channel capacity. The reduction of the code transmission speed per single carrier through the parallel transmission scheme could be implemented by present circuit technology.
The error correcting scheme is, for example, an ARQ (Automatic Repeat reQuest) or FEC (Forward Error Correction) scheme. The parallel transmission scheme is, for example, a multicarrier transmission, CDMA (Code Division Multiple Access), or OFDM (Orthogonal Frequency Division Multiplex) scheme.
The parallel transmission means employing the error correcting scheme has such problems as mentioned below. First, the usefulness of the error correcting scheme causes by increasing the signal bandwidth on the channel or decreasing the channel throughput, constituting an obstacle to effective usefulness of the channel. Second, multiplexing of plural modulated signals causes by increasing the peak power of the combined signal. To settle the first and second problems, studies are being made of them individually.
The first problem is being actively studied in terms of error correcting scheme, and the second problem in terms of a peak power reducing scheme. To reduce the peak power, there have been proposed so far a method of adjusting the initial phase of carrier (Shouichi Narahashi and Toshio Nojima, "Initial Phase Setting Method for Reducing Peak to Average Power Ratio (PAPR) of a Multitone Signal," IECEJ Proc. B-II, No. 11, pp. 663-671, November 1995), a method of searching for combinations of possible signals and taking some measures (Japanese Pat. Appln. Laid-Open No. 504175/94, "Method for Reducing Peak to Average Power Ratio in QAM Communication System"), a method of detecting the peak power and multiplexing signals in a manner to reduce the peak (Shigeru Tomisato, Hiroshi Suzuki, "Envelope Smoothing Parallel Modulation-Demodulation System," IECEJ Technical Bulletin RCS95-77, September 1995), and a method that a signal corresponding to an error correcting symbol is newly sent as a channel for suppressing the peak power (Wilkinson T. A. and Jones A. E., "Minimization of the peak to mean envelope power ratio of multicarrier transmission scheme by block coding," Proc. IEEEE VTS pp. 825-829, 1995). These conventional schemes have, however, defects that the number of carriers for parallel transmission is large and that they cannot be applied to an arbitrary input signal.